Doppler weather radar have experienced great success in locating and calculating precipitation, wind, positions of current weather systems, etc. A Doppler radar transmits a plurality of directional pulses toward an area of interest. The pulses bounce off from various particles, such as precipitation and are then received by a radar receiver as echo pulses. These return echoes are studied based on their intensity, received time, and phase angle relative to the transmitted signal. Generally, there is a compromise between maximizing the unambiguous Doppler velocity (maximum detectable velocity of the target before folding of the signal occurs) and the unambiguous range (maximum distance from the radar to the target before folding of the signal occurs). This is because for a pulse repetition time (PRT) of T, the unambiguous velocity is given by λ/(4T), where λ is the pulse wavelength; whereas the unambiguous range is given by cT/2, where c is the speed of light. The pulse repetition time is the time between transmitted pulses. It is generally desired to have both quantities to be as large as possible. Because the PRT is in the bottom of the equation for the unambiguous velocity, but in the top of the equation for the unambiguous range, there is necessarily a tradeoff between the two.
One method to at least partially overcome this problem is to stagger the spacing of the transmitted pulses. This so called staggered PRT method typically utilizes at least two different alternating pulse spacings T1 and T2. In the case where T1 and T2 have a common interval, i.e. if T1/T2=n/m where n and m are positive integers with no common factors, the common interval is C=T1/n=T2/m, then the unambiguous velocity is given as λ/(4C). The unambiguous range is given by cT2/2, where c is the speed of light in ms−1. It is generally understood that the standard error in the velocity estimate increases as the ratio of T1/T2 approaches unity. Therefore, the ratio is generally chosen as 2/3, however, other ratios are possible, and the particular ratio used should not limit the scope of the invention.
Although the staggered PRT approach overcomes some of the problems experienced by the conflict between the unambiguous velocity and the unambiguous range, the approach can create problems in processing the signals received by the radar. This is particularly true when attempting to suppress the clutter echo in the signal. Signal clutter is produced by echo signals reflected from targets that are not of interest. For example, in weather radar systems, signals reflected off the ground would be an example of clutter. In uniform pulse repetition time systems, clutter filtering can be accomplished according to well known techniques using time domain or spectral domain filtering; almost all prior art techniques rely on uniform pulse repetition times. However, in staggered PRT signal processing, the time series are not uniformly spaced, because of the different spacing between T1 and T2, and thus the aforementioned techniques do not immediately apply.
Prior art approaches to solving this equi-spacing problems include an approach by Sachidananda and Zrnić, (Sachidananda, M. and D. Zrnić, 2002: An Improved Clutter Filtering and Spectral Moment Estimation Algorithm for Staggered PRT Sequences. Journal of Atmospheric and Oceanic Technology, 19, 2009-2019), which is hereby incorporated by reference. This prior art approach introduced a staggered PRT clutter filtering algorithm based on the interpolation of the time-series to equi-spaced data samples. This prior art approach interleaved zeros into the time-series to create equi-spaced time-series data samples. The interpolated time-series is then transformed with a discrete Fourier Transform (DFT). The resulting spectrum contains five modulated “replicas” of the intrinsic underlying spectrum. Complicated matrix mathematics must then be used to filter the spectra and estimate the power, mean velocity, and spectrum width. These complicated calculations result in requiring more sophisticated processing equipment as well as increased processing time.
Therefore, there exists a need for a method to remove clutter from a staggered PRT signal that is easy to implement and can be utilized with existing filters. The present invention solves this and other problems and an advance in the art is achieved.